Since the Great East Japan Earthquake on Mar. 11, 2011, many nuclear power plants have been shut down in terms of the safety. This situation causes a concern about a shortage of power supply, and focuses the interest on alternative renewable energy plants using photovoltaic power generation, wind power generation, geothermal power generation, hydroelectric power generation, tidal power generation, and biomass power generation, instead of nuclear power generation. The photovoltaic power generation, wind power generation, and tidal power generation are expected to be temporary power supplies, but cannot be stable power supplies because of its unstable generation. A small-scale plant of the hydroelectric power generation or tidal power generation is in some demand, whereas a large-scale plant can be built at only limited sites.
The collapse of buildings and forests caused by the Great East Japan Earthquake produced massive woody biomass, such as scrap wood from buildings, fallen trees in forests, timbers remaining in neglected woodlands, and thinnings. Eagerly anticipated is a woody biomass power plant that can effectively utilize such woody biomass. The power plant is also expected to be applied to other biomass than the woody biomass.
A typical woody biomass power plant uses direct combustion power generation or gasification power generation. The direct combustion power generation includes combusting biomass and generating steam with the heat of the combustion, to generate electricity with a steam turbine. The direct combustion power generation can process massive biomass, but the efficiency of the power generation is low. The gasification power generation includes thermally decomposing biomass, and reforming the resulting gas with heat or steam if required, to produce high-calorie gas. The gasification power generation has high efficiency and requires a smaller amount of biomass than that in the direct combustion power generation. Unfortunately, the gasification power generation needs uniform thermal decomposition of biomass and can cause troubles in an apparatus due to the tar generated by the thermal decomposition.
In order to solve the problems in the gasification power generation, for example, Patent Literature 1 (PTL 1) discloses a biomass gasification apparatus including a vertical gasification furnace. Biomass is fed to the upper portion of the gasification furnace to form a moving biomass layer in the gasification furnace, a gasifying agent is fed to the lower portion of the gasification furnace, and the biomass descending in the moving layer is thermally decomposed through the countercurrent contact with the ascending gasifying agent, to produce pyrolyzed gas. The biomass gasification apparatus further includes a vibratory sieve for classifying biomass by size to acquire biomass having an adjusted particle size distribution, which contains biomass particles having a predetermined diameter or smaller at a predetermined weight rate or lower, and a biomass feeder for feeding the biomass having an adjusted particle size distribution from the vibratory sieve to the gasification furnace. The gasification apparatus can ensure uniform upward flow of the high-temperature gas in the moving layer and can reduce the pressure loss in the moving layer, to stabilize the gasification. Unfortunately, the uniform thermal decomposition of the fed biomass is not verified. Furthermore, the necessity of a unit for yielding the biomass having an adjusted particle size distribution leads to an increase in costs.
In order to remove the tar from the pyrolyzed gas, for example, an apparatus for reforming fuel gas in a biomass gasification system (PTL 2) includes a porous heat reservoir, through which fuel gas generated from biomass flows and which is heated to store heat at 1,100° C. or higher, in the flow path of the fuel gas. The system combusts to remove the tar while the fuel gas is passing through the heat reservoir. Unfortunately, the apparatus has a complex configuration and requires complicated manipulation. In addition, part of the fuel gas may also be combusted and lost during the combustion of the tar. Another apparatus for removing the tar from the pyrolyzed gas produced by the thermal decomposition of a biomass material is disclosed in PTL 3, for example. The apparatus includes a compressor for sequentially compressing the pyrolyzed gas from upstream to downstream of the flow of the pyrolyzed gas, and a cooler for cooling the compressed pyrolyzed gas. The apparatus can effectively remove main components, such as furfural, ortho-methoxyphenol, and phenol, of the tar. Unfortunately, the apparatus requires increased facility and operating costs for the compressing and cooling operations. Another pyrolytic gasification system for biomass, such as sewage sludge and woody biomass, is disclosed in PTL 4, for example. The system includes a pyrolytic gasification furnace, a combustion furnace downstream of the pyrolytic gasification furnace, a pipe provided between the pyrolytic gasification furnace and the combustion furnace, an oxidizing agent inlet connected to the pipe to feed an oxidizing agent (mixed gas of inert gas and oxygen) to the pipe, an oxidizing agent adjuster to control the oxygen concentration in the oxidizing agent to 5% to 13% by volume, a heater to heat the inner wall of the pipe, a gas temperature detector to measure the temperature of gas flowing in the pipe, and a gas temperature controller to control the gas temperature. The pyrolytic gasification system can combust to remove the deposits such as tar generated through the thermal decomposition and adhering to the inner wall of the pipe provided between the pyrolytic gasification furnace and the combustion furnace. The system is directed to the quick and safe removal of the pyrolytic deposits derived from biomass. Unfortunately, the system, which combusts the tar generated by the thermal decomposition of the biomass, cannot effectively utilize the tar.
In order to effectively utilize the tar generated by the thermal decomposition of the biomass, for example, a system for reforming woody biomass gas (PTL 5) includes a pyrolytic furnace, a reforming reactor, and an engine. The pyrolytic furnace thermally decomposes introduced woody biomass. The reforming reactor is fed with carbide particles generated by the thermal decomposition in the pyrolytic furnace at the upper portion, and is fed with pyrolyzed gas generated by the thermal decomposition in the pyrolytic furnace at the lower portion, so that the tar vapor contained in the pyrolyzed gas is reformed into hydrogen, methane, and carbon monoxide. The engine is fueled by the reformed gas. The system can effectively utilize the carbide particles (char) remaining after the thermal decomposition. Furthermore, the system reforms the tar with steam into hydrogen, methane, and carbon monoxide, and thus can further improve the gasification efficiency. Unfortunately, the system requires the reforming reactor in addition to the pyrolytic furnace. The system also requires a feeder of carbide particles (char) to the reforming reactor, a circulator of pyrolyzed gas, and feeders of oxygen or air and water. Another biomass carbonizing gas system for effectively utilizing the tar is disclosed in PTL 6, for example. The system thermally decomposes a biomass fuel, such as woody biomass, waste biomass such as urban garbage, and mixed biomass thereof, to carbonize and gasify the biomass fuel. The system includes a carbonizer for heating the biomass fuel to produce carbides, and a two-stage gasification furnace including a high-temperature gasifier for gasifying the carbides and a gas reformer for reforming combustible pyrolyzed gas containing the tar vaporized during the production of the carbides, a carbide feeder for feeding the carbides to the high-temperature gasifier of the gasification furnace, a pyrolyzed gas passage for transferring the combustible pyrolyzed gas generated in the carbonizer to the gas reformer of the gasification furnace, and a gasifying agent feeder. The gasifying agent feeder usually feeds a gasifying agent to the high-temperature gasifier, and feeds a gasifying agent containing oxygen to the gas reformer if the temperature of the exit of the gasification furnace certainly or possibly decreases to a certain level or lower. The system can reduce the amount of generated tar, and can produce high-calorie gas by the reforming operation through the shift reaction. The system can effectively perform the thermal decomposition, the reforming of the pyrolyzed gas, and the decomposition of the tar in sequence. Unfortunately, the system requires the preliminary carbonization of biomass and the feeding of the air for the oxidizing agent, resulting in a decrease in the gasification efficiency.
A typical method of gasifying organic materials such as woody biomass uses heat carriers. For example, PTL 7 discloses a method of producing high-calorie gas from an organic material or organic-material-containing mixture. The heat carriers circulate through a heating zone, a reacting zone, a pyrolyzing zone, a separating zone, and the heating zone again. During the circulation, the organic material or organic-material-containing mixture comes into contact with the heat carriers heated in the pyrolyzing zone and is separated into a carbon-containing residue (solid phase) and pyrolyzed gas (gas phase). After the heat carriers pass through the pyrolyzing zone, the solid carbon-containing residue is separated from the heat carriers through the separating operation. The pyrolyzed gas is mixed with steam serving as a reaction medium, acquires part of the heat of the heat carriers heated in the reacting zone, and thus is further heated, to produce high-calorie gas. The pyrolyzed gas is mixed with steam in the pyrolyzing zone, all the solid carbon-containing residue is transported to another combustion device and is combusted in the combustion device, and hot exhaust gas from the combustion device passes through the heat carriers accumulated in the heating zone such that most of the sensible heat is transferred to the heat carriers. In this method, the mixture is separated into the pyrolytic coke and the heat carriers immediately after exiting a pyrolytic reactor, the resulting pyrolytic coke is combusted in the combustion device, and the sensible heat generated by the combustion is used for heating the heat carriers in the heating zone. The method thus can produce high-calorie gas at low costs. The pyrolytic reactor having the pyrolyzing zone and a gas reforming reactor having the reacting zone are separately provided, so that they can be connected in series or in parallel. In order to stabilize the quality of the pyrolyzed gas while maintaining the thermal efficiency of a preheater for heating the heat carriers in the heating zone, a system (PTL 8) includes a preheater revised from that in the above method. Unfortunately, neither of the method and the system using the heat carriers can sufficiently avoid troubles caused by the tar generated by the thermal decomposition.